1. Field of the Invention
The present invention relates to an imaging device such as a CMOS (Complementary Metal Oxide Semiconductor) image sensor, a method of converting a read-out signal, and a camera.
2. Description of the Related Art
In CMOS image sensors and the like, an analog signal read out from a pixel circuit is converted (A/D converted) into a digital signal (for example, JP-A-2006-303752). In many cases, the A/D conversion is performed by an ADC circuit disposed inside a column processing circuit.
The ADC circuit performs the A/D conversion by comparing a ramp-waveform reference voltage that is generated by a DAC circuit with the voltage of an analog signal read out from the pixel circuit by using a voltage comparator or the like. The column processing circuit is connected to the DAC circuit though several thousands of wirings. Accordingly, there is a problem in that the layout area of the column processing circuit is increased in accordance with an increase in the number of pixels.
In addition, a large current flows by using the DAC circuit in accordance with an increase in the number of pixels. Thus, the driving capability of the DAC circuit may need to be increased. In such a case, the wirings are complicated, and accordingly, there is a problem in that the load of the DAC circuit or the column processing circuit is increased.
As a method of solving the above-described problems, an ADC circuit that performs the A/D conversion by using a voltage-controlled oscillator (hereinafter, referred to as a “VCO”) is disclosed (for example, JP-A-2006-270293).
The ADC circuit using the VCO does not use a voltage comparator. Accordingly, the wiring pattern is simplified, and therefore the above-described problems can be solved.